1. Field of the Invention
The present invention relates to a method of making a fibros thermally insulating layer of coherent structure, in which a quantity of fibers is treated with a binder.
2. The Prior Art
A method of this kind is disclosed in British Pat. No. 2 073 841. In this known method, a pipe perforated in the longitudinal direction is immersed in a very wet slurry consisting of a quantity of fibers and an aqueous binder. One end of the pipe is closed and a vacuum is formed inside the pipe by the application of suction at the other end, so that a very set fiberous layer is formed on the outer wall of the pipe. The pipe covered with this layer is then removed from the slurry and the layer is dried and hardened by heating. After removal from the pipe, a fibrous thermally insulating layer of coherent structure and relatively high hardness is obtained, which can be used independently as an inner pipe for a double-walled (Chimney) pipe element, the outer pipe of which consists of metal.
This known method has a number of drawbacks, to wit:
Since the slurry must be very wet in order to prevent cavities in the required layer, the thickness the layer can attain is very limited because the layer required to be formed on the outer wall of the pipe rapidly closes the perforations therein. The thickness and hence the insulation value of the resulting layer will therefore be low in the radial direction so that its use is limited. In this connection it should be noted that the insulation value of the layer both radially and axially is influenced negatively by the fact that the fibers are pressed closely together during the formation of the layer on the outer wall of the pipe so that the formation of closed air chambers of reasonable size necessary to thermal insulation is obstructed and there is a risk that the fibers will be pressed so close together that the layer can be regarded as consisting of a solid, so that it has low thermal insulation capacity.
Another drawback of the known method is that the resulting thermally insulating layer is hard and therefore liable to injury in respect of fracture. When a layer of this kind is used as a thermally insulating element, special steps must be taken to subject the layer to minimal mechanical loading. In practice, the fixing means required will result in the formation of cold bridges with very low heat resistance.
An additional drawback of the known method consists in that the layer must be formed in a vessel containing the wet fibrous slurry and with a perforate element of a specific shape for coating with the layer, so that the method is unsuitable for making extensive thermally insulating layers and for layers of any arbitrary shape.
Yet another disadvantage of the known method is that it is expensive to use because a considerable amount of heat has to be supplied for drying and hardening the layer.